Just for interest, I've looked through my address book - built up over the past 30 years and consisting of info given to me by the people who live there - and in every case (when they use a number at all) they have written the district name, then the city, then a number by itself.* Both "D" and "Dn" are redundant, and there is no use of it anywhere in the addresses people have given me. I also see that some names have the same postal number, which means that the names are more area-specific.

If you Google "Clontarf Dublin 3" you get a million hits. Google Dn3 and you get Doncaster (UK). Google "Clontarf Dn3" even searching only the Irish pages and you get zilch. What's that tell you?


*
Donnybrook/Ballsbridge
Dublin 4

Rathmines/Ranelagh/Terenure/Rathgar
Dublin 6

Drumcondra
Dublin 9

Ballyfermot
Dublin 10

Baldoyle
Dublin 13

Mulhuddart
Dublin 15

Rathfarnham/Dundrum
Dublin 16

Stepaside
Dublin 18